Abstract

The Nonacho Group is a folded sequence of sedimentary rocks resting on a crystalline basement of probable Archean age. Both sedimentary and basement rocks are intruded by the Sparrow dikes. Fifty-four samples from 10 dikes have a stable magnetization with mean direction after a.f. cleaning of 131°, +51 °(N = 10, k = 50, α95 = 7°) and a corresponding pole at 12 °N, 069 °W. The presence of reversals and the approximate agreement between dikes and their baked contracts are taken to indicate that the dikes were probably magnetized at the time of intrusion. 40Ar–39Ar studies suggest that the probable age of intrusion was about 1700 m.y. The magnetization of the Nonacho sedimentary rocks collected at 12 sites (40 samples) have a mean direction after thermal cleaning of 148°, + 57 °(N = 12, k = 11, α95 = 14°) without correction for geological dip. The corresponding pole is at 13 °N, 086 °W. The precision after correction for geological dip is low (k = 3), so the magnetization is predominantly post-folding. The thermal decay characteristics and the geometry of the directions indicate that the magnetization of the sedimentary rocks was acquired during cooling following a period of mild heating presumably owing to deep burial. This process of secondary magnetization apparently reached its culmination between about 1700 and 1600 m.y., and continued with diminishing intensity until about 1400 m.y. These results are used to modify the apparent polar wandering path for the Canadian Shield, so that the path now has a sharp bend (Hairpin 40) at about 1800 m.y. rather than at 1950 m.y. as previously postulated.